Individual carbon nanotubes (CNTs) are at least one order of magnitude stronger than any other known material. CNTs with perfect atomic structures have a theoretical strength of about 300 GPa. In practice carbon nanotubes do not have perfect structures. However, CNTs that have been prepared have a measured strength of up to about 150 GPa, and the strength may improve upon annealing. For comparison, Kevlar fibers currently used in bullet-proof vests have a strength of only about 3 GPa, and carbon fibers used for making space shuttles and other aerospace structures have strengths of only about 2-5 GPa.
Currently, CNTs prepared according to present methods are far too short to make effective use of their strength and/or cannot be prepared in sufficiently large quantities. A method for synthesizing 40-millimeter long individual carbon nanotubes on silicon substrates, for example, has been reported (see: Zheng et al., Nature Materials, vol. 3, (2004) pp. 673). While the method is useful for preparing CNTs for electronics and sensor applications, it is not useful for preparing CNTs for making structural components because the CNTs cannot easily be removed from the silicon substrate, and the method can't be used to produce CNTs in large enough quantities.
Presently, CNT fibers can be drawn from CNT-polymer solutions or directly from CNT arrays. However, due to practical difficulties in dispersing, assembling and aligning carbon nanotubes using a CNT-polymer route, a strategy based on direct spinning of fibers from CNT arrays is more attractive.
The spinnability of CNT arrays depends greatly on the quality of the arrays, including CNT alignment, density, purity, length, and other factors. Due to difficulties in growing long CNT arrays that are conducive to spinning, CNT yarns so far can only be drawn from arrays of less than 1 mm long, and have a tensile strength lower than 3.3 Pa.
Suitable catalysts are important for synthesizing arrays of long CNTs. Hata et al. for example, prepared such a catalyst and used the catalyst to synthesize an array of long nanotubes (see: Hata et al., Science, (2004), vol. 306, pp. 1362-1364, incorporated by reference). According to Hata et al., the array was synthesized by a water assisted chemical vapor deposition (CVD) procedure using ethylene as a carbon source and a catalyst prepared by sputtering a thin layer of iron on a buffer layer of aluminum oxide. The aluminum oxide layer was previously deposited on the silicon dioxide surface layer of a silicon substrate. This catalyst is typically abbreviated as SiO2/Al2O3(10 nm)/Fe(1 nm), where the positions of the layers in the abbreviation indicate that the aluminum oxide layer is in between the silica layer and the iron layer. Using this catalyst, other researchers have prepared arrays of multi-walled CNTs with CNTs that are less than 2.2 mm in length. Arrays of long multi-walled CNTs can also be obtained using a catalyst structure having a buffer layer of MgO (instead of Al2O3).
There have been efforts to prepare long CNT arrays in the hope CNT fibers with high strength could be spun from these arrays. The longest CNT array (i.e. the CNT array with the longest carbon nanotubes) reported thus far is with CNTs of a length of only 4.0 mm (see: Yun et al., J. Phys. Chem. B, vol. 110, (2006), pp. 23920-23925, incorporated by reference), which is shorter than what will be needed to spin CNT fibers with high strength.
Another problem with known preparations of CNT arrays is the use of a large amount of hydrogen gas in the precursor. Presently, it appears that a feed gas that includes hydrogen in an amount greater than 20 percent and as high as 50 percent hydrogen is required for the growth of long CNT arrays. Hydrogen is relatively expensive and can be dangerous when large amounts are used in the laboratory and industrially. Importantly, CNT arrays of the prior art are generally not good precursors for fibers because they tend to be contaminated with amorphous carbon.
There remains a need for better catalysts, better methods of preparing long CNT arrays, and longer fibers of CNTs with improved strength.